Whether the plural text type is multiple lines of text type column?
A Multiple lines of text column can store up to 63,999 characters.
Please try to use Quick Edit to edit the item then compare the result.
Reference:
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Message like "item is too long" is displayed when I try to edit a list item.
The item is plural text type and has 1573102 characters.
The data can be seen when the item is displayed in a list,
but when I open the item to display/edit,
"item is too long" is displayed instead of the actual data.
The data was migrated to Sharepoint 2019 from Sharepoint 2010 and
the item can be displayed and edited in Sharepoint 2010.
Whether the plural text type is multiple lines of text type column?
A Multiple lines of text column can store up to 63,999 characters.
Please try to use Quick Edit to edit the item then compare the result.
Reference: